Along‐strike segmentation of slow‐slip events (SSEs) and nonvolcanic tremors in Cascadia may reflect heterogeneities of the subducting slab or overlying continental lithosphere. However, the nature behind this segmentation is not fully understood. We develop a 3‐D model for episodic SSEs in northern and central Cascadia, incorporating both seismological and gravitational observations to constrain the heterogeneities in the megathrust fault properties. The 6 year automatically detected tremors are used to constrain the rate‐state friction parameters. The effective normal stress at SSE depths is constrained by along‐margin free‐air and Bouguer gravity anomalies. The along‐strike variation in the long‐term plate convergence rate is also taken into consideration. Simulation results show five segments of ∼Mw6.0 SSEs spontaneously appear along the strike, correlated to the distribution of tremor epicenters. Modeled SSE recurrence intervals are equally comparable to GPS observations using both types of gravity anomaly constraints. However, the model constrained by free‐air anomaly does a better job in reproducing the cumulative slip as well as more consistent surface displacements with GPS observations. The modeled along‐strike segmentation represents the averaged slip release over many SSE cycles, rather than permanent barriers. Individual slow‐slip events can still propagate across the boundaries, which may cause interactions between adjacent SSEs, as observed in time‐dependent GPS inversions. In addition, the moment‐duration scaling is sensitive to the selection of velocity criteria for determining when SSEs occur. Hence, the detection ability of the current GPS network should be considered in the interpretation of slow earthquake source parameter scaling relations.